For many years, display devices such as NIXIE tubes have used mercury vapor along with the normal inert gas content to minimize cathode sputtering. A NIXIE tube comprises a relatively large bulb or envelope in which is mounted a stack of glow cathodes in the form of characters. Mercury is usually introduced in such a device from a glass capsule which is disposed within the tube envelope and contains a ball of mercury. To release the mercury, the capsule is heated to cause the mercury to expand and break the glass.
Recently, thin, flat, gas-filled display panels have come into wide use, and these devices also use an inert gas and mercury vapor. A display panel generally comprises a thin, flat envelope including a base plate and a face plate. The base plate carries various electrodes including groups of glow cathodes, and the face plate carries the associated anodes for the groups of cathodes. The base plate and face plate are sealed together with a close spacing between them. Due to their construction, their gas volume, and the nature of their electrodes, display panels have a relatively strict requirement with respect to the quantity of mercury provided. However, glass capsules cannot be made in mass production so that they uniformly include the same measured quantity of mercury.
In addition, due to limited space in panel devices, the glass capsule cannot be mounted within the tube envelope, so it is mounted in a glass tubulation secured to the base plate of the panel and communicating with the interior of the panel through a hole in the base plate. The tubulation is used to evacuate the panel and to introduce the desired gas filling and mercury vapor. While this arrangement works well, the tubulation represents an item of expense which is considerable when millions of panels are manufactured, and it complicates processing of panels. In addition, the tubulation is fragile and is easily broken.
Another problem in panels arises from the fact that globules of mercury remain in the tubulation after the glass capsule has been broken and after the desired quantity of mercury vapor has entered the panel itself. Often, such globules enter the panel through the hole in the base plate when the panel is handled and during shipping. Such globules can cause problems in the panel, for example, by forming short circuits between closely spaced electrodes.